The high-profile trial of 10 suspects in the multimillion-dollar jewelry heist of Kim Kardashian in Paris in 2016 is underway this week … and three of the defendants in the country’s biggest robbery in decades testified today.

Two of the “Grandpa Robbers” — they’re in their 60s and 70s — took the stand Tuesday, including one who wrote a book about the incident … titled “I Kidnapped Kim Kardashian.”

Yunice Abbas, the 71-year-old author, was up first … fleeing the robbery scene on a bicycle, he fell and dropped a bag of jewelry, spilling gems. He said he picked up what he could, but left a $24,000 diamond necklace on the street. That necklace was found by a pedestrian later that morning, who reportedly wore it at work that day — before finding out about the robbery and turning it over to police, BBC News reports.

You’ll recall … Kim was robbed in her rented apartment at the Hôtel de Pourtalès at about 3 AM on October 3, 2016, during Paris Fashion Week, by two bandits dressed in police outfits, wearing masks … they overpowered a guard, then bound Kim’s hands and feet and left her in her bathtub, before making off with about $10 million worth of jewelry … including a $4-million diamond engagement ring given to her by then-husband Kanye West.

Kim was alone at the time. Prosecutors allege three men were lookouts in the hotel’s reception area, while two other men went upstairs to rob Kim. Eleven people, including a woman, are accused of plotting and executing the crime … one suspect has already died.

Gary Madar — who, at 35, is the youngest of the defendants by decades — was also questioned Tuesday … he’s accused of tipping off the gang to where Kim was staying … his brother had been a driver and provided other transportation services for the Kardashians — including just before Kim and Kanye’s 2014 wedding in Italy. Gary was working for Kourtney Kardashian at the time of the 2016 heist. He denies any involvement in the scheme.

Marc Boyer, a 78-year-old defendant accused of supplying the lone gun used in the robbery, was also questioned, and he talked about exposing his son to a life of crime … his son is also implicated. MB has admitted to being one of the lookouts during the heist.